Synthesis gas, which is also known as syngas, is a mixture of gases comprising primarily carbon monoxide (CO) and hydrogen (H2). Very often it contains carbon dioxide and also Nitrogen. Generally, syngas may be produced from any carbonaceous material. In particular, biomass such as agricultural wastes, forest products, grasses, and other cellulosic material may also be converted to syngas. The composition of syngas highly dependent upon the type of process, feedstock and oxidant used for production. The heating value of syngas is dependent upon the specific chemical composition of the syngas. Gasification-derived syngas differs from the syngas produced from steam methane reforming in terms of composition, calorific value and other contaminants. Syngas produced from air-blown gasifiers typically contains about 50% N2 and 5-20% CO2 and heating value as low as 120 Btu/ft3, which ultimately limits the usage of this gas.
Syngas is a platform intermediate in the chemical and bio-refining industries and has a vast number of uses. Syngas may be used as a feedstock for producing a wide range of chemical products, including liquid fuels, alcohols, acetic acid, dimethyl ether and many other chemical products. These chemicals can be blended into, or used directly as, diesel fuel, gasoline, and other liquid fuels. Syngas can also be directly combusted to produce heat and power. The substitution of alcohols in place of petroleum-based fuels and fuel additives can be particularly environmentally friendly when the alcohols are produced from feed materials other than fossil fuels. However, this syngas needs to be directly produced and converted at the resource site to fuels and/or chemical products since it is not practical to transport the syngas to distant refineries and chemical processing plants.
Improved methods are needed to more cost-effectively produce syngas. Since the syngas generation is a potentially costly step, several alternative processes for syngas generation have been developed.
One alternative process for syngas generation involves the catalytic or thermal reforming reaction between carbon dioxide and methane (typically referred to as dry reforming). An attractive feature of this method is that carbon dioxide is converted into syngas; however, this method has problems with rapid carbon deposition. The carbon deposition or coke forming reaction is a separate reaction from the one that generates the syngas and occurs subsequent to the syngas formation reaction. However, the reaction of methane in dry reforming is slow enough that long residence times are required for high conversion rates and these long residence times lead to coke formation. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 1.0.
A second alternative process for synthesis gas generation involves partial oxidation of methane using oxygen, where the oxygen can be either air, enriched air, or oxygen with a purity in excess of 90%, preferably in excess of 99%. The ratio of hydrogen to carbon monoxide, which is formed from this process, is typically approximately 2.0. However, in commercial practice, some amount of steam is typically added to a partial oxidation reformer in order to control carbon formation and the addition of steam tends to increase the H2/CO ratio above 2.0. Likewise it is common to add relatively small amounts of CO2 to the feed gas mixture in an attempt to adjust the ratio closer to 2.0.
A third approach is to produce syngas with a H2/CO ratio between 0.5 and 1 using a mixture of LPG and CO2 (Calcor process). See, Hydrocarbon Processing, Vol. 64, May 1985, pp. 106-107 and “A new process to make Oxo-feed,” Hydrocarbon Processing, Vol. 66, July 1987, pg. 52. However, many natural gas resource sites, in particular the stranded natural gas sites, do not have the infrastructure available to separate LPG and CO2 from the natural gas.
Following the production of the syngas, many processes and catalysts have been proposed for the production of transportation fuels and chemicals. However, the traditional process for production of fuels and chemicals from syngas involves the production first of a paraffinic wax product that is then refined into fuels and/or chemicals. The refining step is capital intensive and complex to operate, therefore requiring large plant sizes to justify this refining system.
The resources of heavy or low API (American Petroleum Institute) gravity crude oil in the world are more than twice those of conventional light crude oil. Processing of these heavy crude oils provide higher refinery margins. Up-gradation of high residuum content with higher coke forming characteristics through conventional coking processes results in production of significant quantity of low value petroleum coke as by product. Delayed Coking & fluid coking processes produces high amount of low value petroleum coke, which is typically 1.5 times of Concarbon, which means, processing of heavy crude oil having 40% VR with 27% concarbon will produce 16 MT of coke as by product per every 100 MT of crude oil. The price of coke is very low as compared to crude oil price, approximately 1/10th of crude oil price and it erodes the refinery margin heavily. In the current scenario, sustaining the refinery margin in view of ever deteriorating quality of crude oil mainly depends on the profit margin gained from the upgradation of the residue to yield more liquid and less of low value byproducts such as coke.
It is possible to produce higher grade oils through catalytic cracking processes. The feedstock is limited to light residue oils that boil below 550° C. due to excessive regenerator temperatures. In such cases, catalyst coolers are typically used for recovering the extra heat in order to limit the regenerator temperatures. Fresh catalyst make up rate also goes up due to increased catalyst deactivation. Even with the use of catalyst coolers (internal or external cooling) & increased fresh catalyst make up rate, 10 to 30% heavy residue content that boils above 550° C. can be processed along with the light residue oil due to unmanageable coke yield and heat of combustion with the 100% heavy residue content. Further, such catalyst coolers are costly and unreliable.
Various techniques have been proposed in the prior art for handling the extra coke and heat of combustion generated during the cracking of heavy residue content in a circulating fluidized bed. U.S. Pat. Nos. 4,412,914, 4,425,259, 4,450,241, 4,915,820, 6,491,810, 6,585,884, 6,913,687, 7,699,975, 7,744,753, 7,767,075, 7,915,191, 7,932,204, 7,935,245 and 8,518,334 explains the various techniques of gasification of coke deposited during residue feedstock cracking using oxygen containing gases, CO2 and or steam. As these processes use CO2 and or oxygen containing gases as gasifying agent, the syngas produced contains large amount of CO2 and calorific value of the same is very low.
Therefore, there is need of a process which can convert the low value resid streams containing significant amount of concarbon to low boiling point products lean in impurities and high quality synthesis gas rich in hydrogen.